1. Field of the Invention
The invention pertains to automatic steering control of marine vessels, and more particularly to compensation for steering systems that exhibit dead-zone characteristics.
2. Description of the Prior Art
Marine high-speed craft frequently use water jets for combined propulsion and steering. These systems utilize paddles that straddle the water jets. When the paddles are rotated the water streams from the jets are deflected causing the craft to change direction. These devices, however, may exhibit a steering dead-zone. Helm orders for small angular turns produce small angular rotations of the paddles with a concomitant small or zero deflection of the water stream, which may be insufficient to steer the craft. This creates a major control problem, particularly for an autopilot in a course maintaining mode wherein it is continuously making small course corrections. The problem is exacerbated for high speed craft, which are especially prone to instability in maintaining heading due to the dead-zone problem. An uncompensated or improperly compensated dead-zone always results in reduced heading keeping accuracy, longer and less fuel efficient voyages, and a less comfortable ride for passengers and crew.
A dead-zone is characterized by four parameters: the extent of the dead-zone band on either side of zero plus a slope in each direction respectively extending from the ends of the dead-zone. Some prior art marine autopilots for high speed craft utilize the inverse of the dead-zone function to provide dead-zone compensation. These systems, however, require dead-zone parameters which are functions of the speed of the craft, sea conditions, and other factors. At each installation on a vessel, the operator or engineer must go through trial and error searches to establish dead-zone parameters for various operating conditions. The determination of the dead-zone parameters is labor intensive and require trained personnel. Further, during operation the prevailing conditions must be established before the appropriate parameter is applied. The selection of the parameters assumes that the dead-zone is symmetric about zero and the slopes in either direction are known. Additionally, it assumes that the dead-zone characteristic does not change with changes in speed or other vessel changes such as loading. Requiring an operator or engineer to determine the four parameters by trial and error and the operator to constantly update them based on changes in ship operating characteristics is too demanding to be at all practical.
Other prior art system set-up strategies reduce the water jet nozzle diameter to increase the diameter of the water flow, thus providing a greater water stream coverage of the paddles and a greater turning moment and/or offsetting the outboard waterjets in outboard direction so that for small turning angles there is a reduced, but non-zero turning moment from one water jet. Either of these solutions result in reduced propulsion efficiency and performance and, therefore, are not desired by the vessel operator.